Smooth titania nanotubes: Self-organization and stabilization of anatase phase

We present transmission electron microscopy and Raman scattering measurements showing that niobium inhibits the processes of nucleation and growth of anatase crystallites in the initial amorphous titania nanotubes and thus shifts the temperature of the complete amorphous-to-anatase phase transition to higher values up to 550 degrees C. Niobium dopants stabilize the anatase phase in titania nanotubes up to 650 degrees C. The size of anatase crystallites can reach 30-50 nm. Excess niobium atoms which are pulled off from the volume of anatase crystallite form polymeric or monomeric Ti-O-Nb=O groups at the interface area. Slight shift and broadening of E-g (144 cm(-1)), A(1g) (515 cm(-1)) and Eg (630 cm(-1)) modes in Raman spectra can be explained by niobium insertion into the anatase structure. (c) 2008 Published by Elsevier B.V.